1. Field of the Invention The present invention relates to a linear displacement measuring apparatus.
2. Description of Related Art
There have been used displacement measuring apparatuses, that is, what is called encoders in order for various industrial machines to accurately perform position control (for example, JP 2004-301541 A).
A linear displacement measuring apparatus 100 is illustrated in FIG. 1. The linear displacement measuring apparatus 100 includes an elongate scale part 200 and a slider 300 provided so as to be slidable relatively to the scale part 200.
The linear displacement measuring apparatus 100 is mounted to, for example, a moving stage 90. It is assumed that the moving stage 90 is constituted by a base 91 and a stage 92 which is slidable relatively to the base 91. At this time, the scale part 200 is screwed on the side end face of the stage 92, and the slider 300 is screwed on the base 91. With this structure, it is possible to accurately measure the relative displacement of the stage 92 with respect to the base 91.
FIG. 2 is a cross-sectional view taken along the line II-II of FIG. 1.
The scale part 200 includes an elongate main scale 210 (see FIG. 2) and a scale housing case 220 which houses the main scale 210. The main scale 210 is mainly constituted by a glass substrate, and the graduations of a scale are formed in the measurement axis direction. In an example of a photoelectric apparatus, the graduations are equivalent to a diffraction grating.
The scale housing case 220 is hollow and long, and is mainly made of (light) metal, such as aluminum. The scale housing case 220 has a slit 222 along the axial direction on the side face, and the inside and the outside are connected via the slit 222. The main scale 210 is mounted and fixed to the inside of the scale housing case 220. Then, a plurality of holes is drilled in the scale housing case 220 to be mounted and fixed, and the scale housing case 220 is screwed on the stage 92 with the holes.
The slider 300 is provided so as to relatively movable in the longitudinal direction of the scale housing case 220, and detects a relative displacement amount or a relative position with respect to the main scale 210. The slider 300 includes a traveling body 400, a carriage part 310, and coupling means 500. The traveling body 400 travels along the main scale 210 on the main scale 210. The carriage part 310 is provided outside the scale housing case 220 and slides along the scale part 200. The coupling means 500 couples the traveling body 400 with the carriage part 310.
The traveling body 400 includes a detection unit, and the detection unit detects a relative displacement amount with respect to the main scale 210. The coupling means 500 is a joint capable of absorbing angular variation to some extent, and allows the relative displacement between the traveling body 400 and the carriage part 310. (Here, a ball joint is exemplified.)
The moving stage 92 is straightly guided by a linear guide or the like, whereas the main scale 210 made of, for example, glass can have waviness. Unless the relative displacement between the traveling body 400 and the carriage part 310 is allowed to some extent, the traveling body 400 can bite the main scale 210, or the traveling body 400 can float from the main scale 210. Thus, the traveling body 400 and the carriage part 310 are coupled by a joint having a certain degree of freedom. Furthermore, the traveling body 400 is pressed to the main scale 210 by a spring (wire spring) so as to keep a state of facing the main scale 210.
With this structure, when the stage 92 slides, the scale part 200 and the carriage part 310 are relatively displaced accordingly. Since the carriage part 310 and the traveling body 400 are coupled by the coupling means 500, the traveling body 400 as well as the carriage part 310 relatively move with respect to the main scale 210. At this time, the detection unit detects the relative displacement amount with respect to the main scale 210, and outputs the detection to the outside.